Fischer-tropsch synthesis process and system

ABSTRACT

The present invention relates to a Fischer-Tropsch synthesis process and system. The process comprises: a) introducing a feedstock gas containing CO and H 2  into a first stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction; b) separating products of the first stage Fischer-Tropsch synthesis reaction, to separate water from the unconverted tail gas and to obtain hydrocarbon products and the unconverted tail gas; c) introducing the unconverted tail gas obtained in Step b) into a second stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction; d) separating products of the second stage Fischer-Tropsch synthesis reaction, to separate water from the unconverted tail gas, with a portion of the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction being returned to the second stage Fischer-Tropsch synthesis reactor for recycle reactions. The process and system are suitable for large-scale industrialized production.

TECHNICAL FIELD

The present invention relates to a Fischer-Tropsch synthesis process and system, particularly to a two-stage Fischer-Tropsch synthesis process and system.

BACKGROUND ART

As the price of oil rises increasingly in recent years, people place a higher emphasis on the technology for developing and producing alternative oil products. They produce synthetic gases using coal, natural gas or other substances, process synthetic gases underwent water gas transformation and synthetic gas purification technology according to the requirements of Fischer-Tropsch synthesis catalysts for synthetic gases, take the processed synthetic gases as feedstocks to produce hydrocarbons and the byproducts oxygen-containing compounds through Fischer-Tropsch synthesis, and then process them using mature petroleum processing techniques to produce high-quality environment-friendly oil products. The core of the process is Fischer-Tropsch synthesis techniques. Thus, it can be seen that to develop new Fischer-Tropsch synthesis techniques is of great significance for developing production techniques of alternative oil products.

Coal Liquefication Technology (authors: Gao Jinsheng, Zhang Dexiang, Chemical Industry Press, March 2005, Edition 1) describes Sasol's mature Fischer-Tropsch synthesis process. The process is fundamentally a single-stage process. According to reactor types, the process is divided into the Arge process using fixed beds, the Kellogg process using fluidized beds, and the Sasol slurry bed Fischer-Tropsch synthesis process using slurry beds. In Sasol Plant I, tail gas of the Arge process is selected as feedstock of the Kellogg process, and there is no two-stage technical process using the same reactor.

Slurry bed reactor is a triphase bubbling column, operated in medium temperatures, wherein the feedstock gas (gas phase) bubbles in molten Fischer-Tropsch synthetic wax (liquid phase) and granules of catalysts (solid phase). Feedstocks of preheated synthetic gas enter the reactor from the bottom of the reactor, and diffuse into the slurry composed by the generated liquid wax and granules of catalysts. During the process of the bubbles rising upward, the synthetic gas constantly goes through Fischer-Tropsch synthesis reactions to generate more Fischer-Tropsch synthesis waxes. The heat generated from the reaction is taken out by the steam produced by the built-in cooling coil. The wax products are separated in the built-in filter of the reactor slurry bed reaction zone to get out of the reactor, or the wax products are obtained in the method of separating Fischer-Tropsch synthesis waxes from solid particulates of catalysts with a solid-liquid separation device laid outside after the slurry is extracted out. After the tail gas coming out from the top of the reactor is cooled, light components and water can be recovered. The obtained hydrocarbon products are sent to a downstream product-refining device, and the water is sent to a recovery device for disposal.

The slurry bed reactor possesses a good heat transfer carry outance, enables the reactants in it mix well, is favorable for controlling reaction temperature and moving reaction heat out, and enables isothermal operation, thereby obtaining a higher reaction rate at a higher average operating temperature; the reactor features simple control and low operation costs; by replacing catalysts regularly, the average lifespan of catalysts is easy to control; the selectivity of the process is even easier to control, thereby improving the quality of intermediate products. Therefore, when synthetic wax and diesel oil are mainly produced, the slurry bed Fischer-Tropsch synthesis has obvious advantages, and becomes a development trend of the Fischer-Tropsch synthesis technology.

The slurry bed reactor is designed uniquely. In order to keep the characteristics of a slurry bed, firstly, the catalysts shall be produced into fine granules for suspending in a liquid phase zone; secondly, the operation shall be carried out in a specific reaction temperature range, so that the Fischer-Tropsch synthesis waxes generated from the reaction exist in a liquid phase to provide slurry conditions. In addition, the operation shall be carried out at a certain gas flow rate (superficial gas velocity), thus preventing granules of catalysts from sedimentation due to a too low gas flow rate, and preventing the gas from carrying granules of catalysts out of the reactor due to a too high gas flow rate. Therefore, when there is a need to improve the productivity of the Fischer-Tropsch synthesis device, i.e. to increase the gas flow volume of feedstocks of the Fischer-Tropsch synthesis, the diameter of the reactor shall be increased to maintain the gas speed required for an empty column.

However, when there is a need to improve the capability of the device for producing Fischer-Tropsch synthetic oil to more than 500,000 tons/year, limited by reactor manufacturing technologies, transport conditions, etc., the maximum diameter of a reactor at present can only reach around 10 meters. Therefore, at least two slurry bed reactors are required for the processing. Currently the megaton Fischer-Tropsch synthetic device that has been put into industrialized production is designed as two sets of complete conventional single-set slurry bed reactors with Fischer-Tropsch synthesis process and parallel operations. Oil-water-gas separation systems are provided for products at the top of each reactor, most of the tail gas is recycled back to each reactor to obtain the required total carbon conversion rate, and a small portion of the tail gas is vented out.

Chinese Patent Publication No. CN1611565 (application No.: CN200310108146.X) discloses a process for producing liquid fuels using a synthetic gas, comprising a Fischer-Tropsch synthesis unit, a C₃-C₅ recovery unit, and an oil-refining unit. The Fischer-Tropsch synthesis unit is divided into two stages, and all products thereof are wax and condensates; the two stages of Fischer-Tropsch synthesis devices respectively recycle partial tail gas; the remaining tail gas of a first stage Fischer-Tropsch synthetic gas is introduced into a second stage Fischer-Tropsch synthesis device as feedstock gas; the remaining tail gas of the second stage Fischer-Tropsch synthesis device is introduced into the C₃-C₅ recovery unit. The C₃-C₅ recovery unit recovers most of the components above C₃ in the tail gas in the method of in-depth cooling, and the components enter the oil-refining unit together with the waxes and condensates produced by the two stages of Fischer-Tropsch synthesis units to produce liquid fuels. This process adopts two-stage device Fischer-Tropsch synthesis, which can lower CH₄ productivity in the method of reducing the single-stage CO conversion rate. However, in this process, partial tail gas in the first stage Fischer-Tropsch synthesis device is recycled, which increases the investment and operation expenses of the recycle compressor system and lowers the economical efficiency of the whole process. In addition, the first stage Fischer-Tropsch synthesis device adopts partial tail gas recycling, which reduces the load of the reactor for processing feedstocks of fresh synthetic gas, and lowers the oil yield.

Chinese Patent Publication No. CN1948438 (application No.: CN200610140020.4) relates to a Fischer-Tropsch synthesis method, comprising the following steps: a) introducing feedstocks of a synthetic gas into a first stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts; b) separating the products of the first stage Fischer-Tropsch synthesis reaction, with a portion of tail gas being returned to the first stage Fischer-Tropsch synthesis reactor for recycle reactions, and C₁-C₄ hydrocarbons contained in other tail gases being converted to synthetic gases; c) mixing the converted tail gas from Step b) with the recycled tail gas from the second stage Fischer-Tropsch synthesis reaction, introducing the mixed gas into the second stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts; d) separating the products of the second stage Fischer-Tropsch synthesis reaction, with most of the tail gas being returned to the second stage Fischer-Tropsch synthesis reactor for recycle reactions, and the remaining tail gas being vented out. In this method, a gas recycling process is adopted and a gas recycle compressor is provided in the first stage, which lowers the capability of the reactor for processing fresh synthetic gases; moreover, when the tail gas is introduced into the second stage, the process for producing natural gas into synthetic gas through self-heating oxidation is required to convert C₁-C₄ light hydrocarbons to synthetic gases, which increases the costs of devices.

Chinese Patent Publication No. CN100575457C (patent No.: ZL200610140019.1) relates to a Fischer-Tropsch synthesis method, comprising the following steps: (1) introducing feedstocks of coals for producing synthetic gases into a first stage Fischer-Tropsch synthesis reactor to contact iron-based catalysts, and making them react under the Fischer-Tropsch synthesis reaction conditions; (2) separating the products of the first stage reaction, with the tail gas left after the reaction being removed CO₂ and entering a C1-C4 hydrocarbon conversion device to produce CO and H₂, introducing the converted tail gas into a second stage Fischer-Tropsch synthesis reactor to contact and react with cobalt-based catalysts under the Fischer-Tropsch synthesis reaction conditions; (3) separating the products of the second stage reaction, venting a portion of the tail gas, with other tail gases being returned to the first stage Fischer-Tropsch synthesis reactor for recycled use. In this method, a slurry bed is used for the first stage reactor, and a portion of the feed gas is the recycled gas of tail gas of the second stage reaction. Cobalt-based catalysts are used for the second stage reaction, although this reaction has a higher space time yield, taking heat from reaction is hard to control, as the reactor is a fixed bed. Moreover, the fixed bed process is hard to use in a large scale.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a Fischer-Tropsch synthesis process and system, particularly a two-stage Fischer-Tropsch synthesis process and system.

An aspect of the present invention relates to a Fischer-Tropsch synthesis process, comprising the following steps:

a) a first stage Fischer-Tropsch synthesis reaction

introducing a feedstock gas containing CO and H₂ into a first stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts, to obtain the products of the first stage Fischer-Tropsch synthesis reaction;

b) separation of the products of the first stage Fischer-Tropsch synthesis reaction

separating the products of the first stage Fischer-Tropsch synthesis reaction, so as to separate water from the unconverted tail gas and to obtain hydrocarbon products and unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction;

c) a second stage Fischer-Tropsch synthesis reaction

introducing the unconverted tail gas obtained from Step b) into a second stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts, to obtain products of the second stage Fischer-Tropsch synthesis reaction;

d) separation of the products of the second stage Fischer-Tropsch synthesis reaction

separating the products of the second stage Fischer-Tropsch synthesis reaction, so as to separate water from the unconverted tail gas and to obtain hydrocarbon products and unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction, with a portion of the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction being returned to the second stage Fischer-Tropsch synthesis reactor for recycle reactions.

Another aspect of the present invention relates to a Fischer-Tropsch synthesis system, comprising:

A) a first stage Fischer-Tropsch synthesis reactor, containing Fischer-Tropsch synthesis catalysts, the first stage Fischer-Tropsch synthesis reactor at least comprising:

a first stage reactor inlet, located at the bottom of the first stage Fischer-Tropsch synthesis reactor;

a first stage reactor top outlet, located at the top of the first stage Fischer-Tropsch synthesis reactor;

a first stage Fischer-Tropsch synthesis wax or slurry outlet, located at a slurry bed zone of the first stage Fischer-Tropsch synthesis reactor;

B) a first stage separation system, for separating the top products of the first stage reactor top outlet, thereby separating water from the unconverted tail gas to obtain hydrocarbon products and the unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction; the first stage separation system comprising:

a first stage separation system inlet, connected with a first stage reactor top outlet;

a plurality of first stage separation system outlets, comprising:

a first stage hydrocarbon product outlet, and

a first stage unconverted tail gas outlet;

C) a second stage Fischer-Tropsch synthesis reactor, containing Fischer-Tropsch synthesis catalysts, the second stage Fischer-Tropsch synthesis reactor at least comprising:

a second stage reactor inlet, located at the bottom of the second stage Fischer-Tropsch synthesis reactor, and connected with a first stage unconverted tail gas outlet;

a second stage reactor top outlet, located at the top of the second stage Fischer-Tropsch synthesis reactor;

a second stage Fischer-Tropsch synthesis wax or slurry outlet, located at a slurry bed zone of the second stage Fischer-Tropsch synthesis reactor;

D) a second stage separation system, for separating top products of the second stage reactor top outlet, thereby separating water from the unconverted tail gas to obtain hydrocarbon products and the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction, the second stage separation system comprising:

a second stage separation system inlet, connected with a second stage reactor top outlet;

a plurality of second stage separation system outlets, comprising:

a second stage hydrocarbon product outlet, and

a second stage unconverted tail gas outlet.

The process and system of the present invention overcome the defects of the conventional Fischer-Tropsch synthesis process for producing liquid products, such as high device investment and operation expenses, low space time yield, and low CO utilization ratio, simplify the technical process, and are suitable for large-scale industrial production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart for a two-stage large-scale Fischer-Tropsch synthesis process in the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is carried out based on the understanding about the fundamental features of the Fischer-Tropsch synthesis reaction (particularly Fischer-Tropsch synthesis reaction of iron-based catalysts).

The Fischer-Tropsch synthesis reaction of iron-based catalysts is divided into two types:

CO+H₂→HC (hydrocarbon)+H₂O  (1)

CO+H₂O

CO₂+H₂  (2)

wherein the second type reaction (2) is a water gas shift reaction (WGS reaction), and it is a reversible reaction, the CO conversion thereof accounts for 15-35% of the total carbon conversion.

Obviously, the method for improving the Fischer-Tropsch synthesis reaction rate, increasing the capability of the device for producing Fischer-Tropsch synthetic oil, and improving CO utilization ratio lies in increasing the flow volume of the effective synthetic gas (CO+H₂) at the reactor inlet (can be represented by partial pressure of the effective synthetic gas when a specific superficial gas velocity is maintained), and reducing WGS forward reaction velocity. It can be seen from the WGS reaction characteristics that, there are two effective means for reducing WGS forward reaction velocity and improving CO utilization ratio: a. decreasing H₂O partial pressure in the system; b. improving CO₂ partial pressure in the system.

On the other hand, as the second type of WGS reactions exists on iron-based catalysts, reducing water partial pressure in the system is favorable for decreasing WGS forward reaction velocity, which relatively increases the concentration of CO reactants that are converted to hydrocarbons in the first type of Fischer-Tropsch synthesis reactions, and is favorable for improving the Fischer-Tropsch synthesis reaction rate.

The inventor finds that when the reactor is arranged in stages and gas phase removing is carried out between the stages, Fischer-Tropsch synthesis reaction rate can be improved, and productivity of the device can be increased.

When cobalt-based catalysts exist, although WGS reaction seldom occurs, the presence of water produced from the Fischer-Tropsch synthesis reaction will also affect the Fischer-Tropsch synthesis reaction rate.

The present invention is provided based on the understanding about the features of the Fischer-Tropsch synthesis reaction, and the tests for the same.

In the present invention, the terms “connected with . . . ” and “connected to” mean that two objects connect directly or via common components or devices (e.g. a valve, a pump, a heat exchanger).

If without confliction, all the Examples, embodiments and features of the present invention can be combined with each other.

The present invention relates to a Fischer-Tropsch synthesis process, comprising the following steps:

a) a first stage Fischer-Tropsch synthesis reaction

introducing a feedstock gas containing CO and H₂ into a first stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts, to obtain products of the first stage Fischer-Tropsch synthesis reaction;

b) separation of the products of the first stage Fischer-Tropsch synthesis reaction

separating the products of the first stage Fischer-Tropsch synthesis reaction, so as to separate water from the unconverted tail gas and to obtain hydrocarbon products and unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction;

c) a second stage Fischer-Tropsch synthesis reaction

introducing the unconverted tail gas obtained from Step b) into a second stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts, to obtain products of the second stage Fischer-Tropsch synthesis reaction;

d) separation of the products of the second stage Fischer-Tropsch synthesis reaction

separating the products of the second stage Fischer-Tropsch synthesis reaction, so as to separate water from the unconverted tail gas and to obtain hydrocarbon products and unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction, with a portion of the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction being returned to the second stage Fischer-Tropsch synthesis reactor for recycle reactions.

Preferably, unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction is not returned to the first stage Fischer-Tropsch synthesis reactor for recycle reactions.

Preferably, in Step a), the fresh synthetic gas as the feedstock gas passes through the first stage Fischer-Tropsch synthesis reactor in one way.

Preferably, control the CO conversion rate in the first stage Fischer-Tropsch synthesis reactor at 30%-70%, more preferably at 40%-65%, and further preferably at 50%-60%.

Preferably, the water content in the feedstock gas is less than 0.05%, preferably less than 0.01%, more preferably less than 0.005%, and most preferably less than 0.0001% (volume ratio).

Preferably, the water content in the unconverted tail gas entering the second stage Fischer-Tropsch synthesis reactor is less than 0.05%, preferably less than 0.01%, more preferably less than 0.005%, and most preferably less than 0.0001% (volume ratio).

Preferably, control the CO conversion rate in the first stage Fischer-Tropsch synthesis reactor at 30%-70%, more preferably at 40%-65%, and further preferably at 50%-60%.

The separations in Steps b) and d) comprise oil-water-gas separations of top products in Fischer-Tropsch synthesis reactions. Preferably, oil-water-gas separations of top products in the first stage Fischer-Tropsch synthesis reaction in Step b) and/or oil-water-gas separations of top products in the second stage Fischer-Tropsch synthesis reaction in Step d) comprise the following steps:

firstly, carrying out flash separation using a thermal high-pressure separator (“thermal high separator” for short) to obtain thermal high-pressure separator liquids and gases;

then, carrying out flash separation for the thermal high pressure separator gases using a cold high pressure separator (“cold high separator” for short) to obtain two phases: cold high pressure separator liquid being a blended liquid phase product of light distillate oil and water, and cold high pressure separator gas being unconverted tail gas.

Preferably, the thermal high-pressure separator is operated at 120-220° C., preferably at 140-180° C. Preferably, the cold high-pressure separator is operated at 5-60° C., more preferably at 10-50° C.

Preferably, the process of the present invention further comprises:

e) introducing the thermal high pressure separator liquids of the first stage and second stage Fischer-Tropsch synthesis reactions to a thermal low pressure separator (“thermal low separator” for short), and carrying out flash separation again to obtain heavy distillate oil products and thermal low pressure separator gases; and optionally

f) introducing the blended liquid phase products of the cold high pressure separator (“cold high separator” for short) of the first stage and second stage Fischer-Tropsch synthesis reactions and optional thermal low pressure separator gases into a cold low pressure separator (“cold low separator” for short), and carrying out flash separation to obtain cold low pressure separator gases, light distillate oil products and water.

The thermal low-pressure separator can run at 60-200° C., preferably at 70-180° C., more preferably at 80-160° C., and most preferably at 90-140° C. The cold low-pressure separator can run at 5-60° C., preferably at 20-50° C.

Preferably, the first stage Fischer-Tropsch synthesis reaction in Step a) is carried out under the following reaction conditions.

Preferably, as to the first stage Fischer-Tropsch synthesis reaction in Step a), the reaction temperature is 200-320° C., preferably 235-275° C., and more preferably 245-265° C.

Preferably, as to the first stage Fischer-Tropsch synthesis reaction in Step a), the reaction pressure is 15-50 bar, preferably 20-40 bar, and more preferably 25-35 bar.

Preferably, as to the first stage Fischer-Tropsch synthesis reaction in Step a), the superficial gas velocity of reactor inlet gas is 10-40 cm/s, preferably 15-35 cm/s, and more preferably 15-25 cm/s.

Preferably, as to the first stage Fischer-Tropsch synthesis reaction in Step a), the ratio (i.e. gas catalyst ratio) of volumetric flow of the feedstock gas (the reactor inlet gas) to the mass of catalyst is 2000-50000 Nml/g-cat./h, preferably 5000-30000 Nml/g-cat./h, and more preferably 8000-20000 Nml/g-cat./h.

Preferably, the second stage Fischer-Tropsch synthesis reaction in Step c) is carried out under the following reaction conditions.

Preferably, as to the second stage Fischer-Tropsch synthesis reaction in Step a), the reaction temperature is 200-320° C., preferably 235-275° C., and more preferably 245-265° C.

Preferably, as to the second stage Fischer-Tropsch synthesis reaction in Step a), the reaction pressure is 15-50 bar, preferably 18-38 bar, and more preferably 25-35 bar.

Preferably, as to the second stage Fischer-Tropsch synthesis reaction in Step a), the superficial gas velocity of the reactor inlet is 10-40 cm/s, preferably 15-35 cm/s, and more preferably 15-25 cm/s.

Preferably, as to the second stage Fischer-Tropsch synthesis reaction in Step a), the volume ratio (i.e. gas catalyst ratio) of gas flow to catalyst at the reactor inlet is 2000-50000 Nml/g-cat./h, preferably 5000-30000 Nml/g-cat./h, and more preferably 8000-20000 Nml/g-cat./h.

Preferably, as to the first stage Fischer-Tropsch synthesis reaction in Step a), the volume ratio of CO to H₂ in the feedstock gas is 0.67-2.2, preferably 0.8-2, more preferably 1-2, and most preferably 1.4-2.

Preferably, the feedstock gas in Step a) is a synthetic gas, and is preferably a synthetic gas underwent purification and water-gas shift. Fresh synthetic gas is preferred as the feedstock gas.

Preferably, the catalysts used in Steps a) and c) are iron-based or cobalt-based catalysts. Preferably, when iron-based catalysts are adopted, the volume ratio of H₂ to CO in the feedstock gas is 1.4-1.8, preferably 1.4-1.7, more preferably 1.5-1.7, and most preferably 1.5-1.6. Preferably, when cobalt-based catalysts are adopted, the volume ratio of H₂ to CO in the feedstock gas is 1.8-2.2, preferably 1.9-2.1, more preferably 1.95-2.05, and most preferably 2.0.

Preferably, the number of the first stage Fischer-Tropsch synthesis reactor is equal to or more than the number of the second stage Fischer-Tropsch synthesis reactor, more preferably, the number of the first stage Fischer-Tropsch synthesis reactor is more than the number of the second stage Fischer-Tropsch synthesis reactor. When the number of the first stage Fischer-Tropsch synthesis reactor or the number of the first stage Fischer-Tropsch synthesis reactor is more than one (i.e. more than two), the Fischer-Tropsch synthesis reactors of this stage are in parallel.

Preferably, the first stage Fischer-Tropsch synthesis reactor and the second stage Fischer-Tropsch synthesis reactor are slurry bed reactors. Preferably the first stage Fischer-Tropsch synthesis reactor is one slurry bed reactor or a plurality of slurry bed reactors in parallel, and the second stage Fischer-Tropsch synthesis reactor is one slurry bed reactor. Also, Fischer-Tropsch synthesis reactors can be fixed beds, fixed fluidized beds, and fluidized beds.

Preferably, the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction is blended with all the unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction, and then is introduced into the second stage Fischer-Tropsch synthesis reactor. Preferably, the blend volume ratio of the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction to the unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction is 0.5-5, preferably 1-3, and more preferably 1.5-2.5. Preferably, CO₂ in the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction is removed before the blending. For example, removing of CO₂ is carried out with hot potassium carbonate CO₂-removing process or in water washing method.

Preferably, in Step b), top products of the reactor are introduced into the thermal high-pressure separator (e.g. at 120-220° C., preferably at 140-180° C.), to carry out flash separation to separate gas phase from heavy oil phase. Gas phase streams are introduced into the cold high-pressure separator (e.g. at 5-60° C., preferably at 10-50° C.), to carry out flash separation again to obtain blended liquid phase products of gas, light distillate oil and synthetic water.

Preferably, in Step d), top products of the reactor are introduced into the thermal high-pressure separator (e.g. at 120-220° C., preferably at 140-180° C.), to carry out flash separation to obtain gas phase and heavy oil phase. Gas phase streams are introduced into the cold high-pressure separator (e.g. at 5-60° C., preferably at 10-50° C.), to carry out flash separation again to obtain blended liquid phase products of gas, light distillate oil and synthetic water.

The discharge amount of tail gas is determined by the contents of CO and H₂ in the tail gas to ensue that the contents of CO and H₂ in the tail gas are not less than 40%, when the contents of CO and H₂ in the tail gas are greater than 50%, the discharge amount shall be decreased; when the contents of CO and H₂ in the tail gas are less than 40%, the discharge amount shall be increased.

Another aspect of the present invention relates to a Fischer-Tropsch synthesis system, comprising:

A) a first stage Fischer-Tropsch synthesis reactor, containing Fischer-Tropsch synthesis catalysts, the first stage Fischer-Tropsch synthesis reactor comprising:

a first stage reactor inlet, located at the bottom of the first stage Fischer-Tropsch synthesis reactor;

a first stage reactor top outlet, located at the top of the first stage Fischer-Tropsch synthesis reactor;

a first stage Fischer-Tropsch synthesis wax or slurry outlet, located at a slurry bed zone of the first stage Fischer-Tropsch synthesis reactor;

B) a first stage separation system, for separating top products of the first stage reactor top outlet, thereby separating water from the unconverted tail gas to obtain hydrocarbon products and unconverted tail gas of the first stage Fischer-Tropsch Synthesis reaction, the first stage separation system comprising:

a first stage separation system inlet, connected with the first stage reactor top outlet;

a plurality of first stage separation system outlets, comprising:

a first stage hydrocarbon product outlet, and

a first stage unconverted tail gas outlet;

C) a second stage Fischer-Tropsch synthesis reactor, containing Fischer-Tropsch synthesis catalysts, the second stage Fischer-Tropsch synthesis reactor comprising:

a second stage reactor inlet, located at the bottom of the second stage Fischer-Tropsch synthesis reactor, and connected with the first stage unconverted tail gas outlet;

a second stage reactor top outlet, located at the top of the second stage Fischer-Tropsch synthesis reactor 102;

a second stage Fischer-Tropsch synthesis wax or slurry outlet, located at a slurry bed zone of the second stage Fischer-Tropsch synthesis reactor;

D) a second stage separation system, for separating top products of the second stage reactor top outlet, thereby separating water from the unconverted tail gas to obtain hydrocarbon products and unconverted tail gas of the second stage Fischer-Tropsch Synthesis reaction, the second stage separation system comprising:

a second stage separation system inlet, connected with a top outlet of the second stage reactor;

a plurality of second stage separation system outlets, comprising:

a second stage hydrocarbon product outlet, and

a second stage unconverted tail gas outlet.

Preferably, the first stage unconverted tail gas outlet is not connect with the first stage reactor inlet.

Preferably, the first stage separation system in B) and/or the second stage separation system in D) comprise an oil-water-gas separation device.

Preferably, the oil-water-gas separation system of the first stage separation system in B) and/or the oil-water-gas separation system of the second stage separation system in D) comprises:

a thermal high-pressure separator, comprising:

a thermal high-pressure separator inlet, connected with the first stage separation system inlet or the second stage separation system inlet;

a thermal high-pressure separator liquid outlet, and

a thermal high-pressure separator gas outlet;

a cold high-pressure separator, comprising:

a cold high-pressure separator inlet, connected with the thermal high-pressure separator gas outlet;

a cold high-pressure separator liquid outlet, and

a cold high-pressure separator gas outlet.

Preferably, the Fischer-Tropsch synthesis system of the present invention further comprises:

a thermal low-pressure separator, comprising:

a thermal low-pressure separator inlet, connected with the thermal high-pressure separator liquid outlet of the first stage separation system and/or the second stage separation system,

a thermal low-pressure separator gas outlet,

a thermal low-pressure separator liquid outlet; optionally, a cold low-pressure separator, comprising:

a cold low-pressure separator inlet, connected to the thermal low-pressure separator liquid outlet and/or the cold high-pressure separator liquid product outlet,

a cold low-pressure separator gas outlet,

a light distillate oil outlet,

a Fischer-Tropsch synthesis water outlet.

Preferably, the Fischer-Tropsch synthesis system of the present invention further comprises a CO₂-removing system, which comprises:

a CO₂-removing solvent inlet,

a CO₂-removing solvent outlet,

a gas inlet of the CO₂-removing system, connected with a gas outlet of the cold high-pressure separator,

a gas outlet of the CO₂-removing system, connected with a second stage reactor inlet.

Preferably, the first stage Fischer-Tropsch synthesis reactor and the second stage Fischer-Tropsch synthesis reactor is one slurry bed reactor or a plurality of slurry bed reactors in parallel; preferably, the first stage Fischer-Tropsch synthesis reactor is one slurry bed reactor or a plurality of slurry bed reactors in parallel, while the second stage Fischer-Tropsch synthesis reactor is one slurry bed reactor.

Preferably, the number of the first stage Fischer-Tropsch synthesis reactor is more than or equal to the number of the second stage Fischer-Tropsch synthesis reactor, and preferably more than the number of the second stage Fischer-Tropsch synthesis reactor.

Preferably, the Fischer-Tropsch synthesis system of the present invention further comprises a wax filter, arranged inside or outside the first stage Fischer-Tropsch synthesis reactor and the second stage Fischer-Tropsch synthesis reactor.

Preferably, the Fischer-Tropsch synthesis system of the present invention further comprises a Fischer-Tropsch synthesis wax outlet, arranged at the central slurry zone of the first stage Fischer-Tropsch synthesis reactor and the second stage Fischer-Tropsch synthesis reactor.

Preferably the process and system of the present invention is a two-stage Fischer-Tropsch synthesis process and system.

The process of the present invention is simply described as follows: as for megaton large-scale industrialized device, as more than two reactors are required, the fresh synthetic feedstock gas passes through a first stage Fischer-Tropsch synthesis reactor in one way, top products of the first stage Fischer-Tropsch synthesis reactor undergo oil-water-gas separation, the obtained unconverted tail gas is introduced into a second stage Fischer-Tropsch synthesis reactor, top outlet products of which undergo oil-water-gas separation once again, after that, most of the tail gas recycles back to the second stage Fischer-Tropsch synthesis reactor to obtain the required total CO conversion rate. This technical device is highly integrated, all liquid phase products obtained from the thermal high-pressure separators in the two stages of reactions enter a same thermal low-pressure separator for flash separation once again to obtain heavy distillate oil, while all liquid products (mixtures of light distillate oil and synthetic water) of the cold high-pressure separators in the two stages of reactions flow to a same cold low-pressure separator for separation, thus obtaining light distillate oil and Fischer-Tropsch synthesis water products.

In a preferred embodiment, the two-stage large-scale Fischer-Tropsch synthesis process provided in the present invention comprises the following steps:

a) introducing the feedstock gas of fresh synthetic gas into a first stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts;

b) carrying out flash separation for top products of the first stage Fischer-Tropsch synthesis reactor using a thermal high-pressure separator and a cold high-pressure separator consecutively to obtain heavy oil phase, blended liquid phase of light oil and synthetic water, unconverted tail gas, and likes as first stage Fischer-Tropsch synthesis reaction products;

c) blending the cold high-pressure separator gas of the first stage Fischer-Tropsch synthesis with the recycled tail gas of the second stage Fischer-Tropsch synthesis reaction, and introducing the blended gas into the second stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts;

d) carrying out flash separation for the second stage Fischer-Tropsch synthesis reaction products using a thermal high-pressure separator and a cold high-pressure separator consecutively to obtain heavy oil phase, blended liquid phase of light oil and synthetic water, tail gas, etc., with most tail gas being returned to the second stage Fischer-Tropsch synthesis reactor for recycle reactions, and other tail gases being vented out;

e) introducing thermal high-pressure separator liquids of the first stage and second stage Fischer-Tropsch synthesis simultaneously to a thermal low-pressure separator, carrying out flash separation again to obtain heavy distillate oil products;

f) introducing blended liquid phase products of cold high-pressure separators of the first stage and second stage Fischer-Tropsch synthesis simultaneously to a cold low-pressure separator, carrying out flash separation again to obtain light distillate oil and synthetic water products.

The two-stage Fischer-Tropsch synthesis process of the present invention will be further elaborated in detail in the following pages, but the present invention is not therefore subject to any limitations.

The fresh synthetic gas in Step a) of the above Fischer-Tropsch synthesis process can be produced from coal, natural gas or organic substances, but it must be purified to remove sulfur and other substances contained therein that are harmful to Fischer-Tropsch synthesis catalysts. Coal Liquefication Technology (Chemical Industry Press, authors: Gao Jinsheng and Zhang Dexiang, published on March 2005) describes in detail the technologies related with synthetic gas production, purification and shift. The volume ratio of H₂ to CO in fresh synthetic gas is 0.67-2.2, preferably 0.8-2, more preferably 1-2, and most preferably 1.4-2.

The first stage Fischer-Tropsch synthesis reaction in Step a) is carried out under the following reaction conditions: the reaction temperature is 200-320° C., preferably 235-275° C.; the reaction pressure is 15-50 bar, preferably 20-40 bar; superficial gas velocity of reactor inlet gas reaches 10-40 cm/s, preferably 15-35 cm/s; the volume ratio of feedstock of fresh synthetic gas to catalyst is 2000-50000, preferably 5000-30000. The present invention belongs to low-temperature Fischer-Tropsch synthesis reactions.

In Step b), top products of the reactor enter a thermal high-pressure separator, and carry out flash separation at 120-220° C., preferably 140-180° C., so as to separate gas phase (thermal high-pressure separator gas) from heavy oil phase (thermal high-pressure separator liquid). Gas phase streams enter the cold high-pressure separator, and carry out flash separation again at 5-60° C., preferably 10-50° C. to obtain blended liquid phase products (cold high pressure separator liquids) of gas, light distillate oil and synthetic water.

In Step c), the recycled tail gas of the second stage Fischer-Tropsch synthesis reaction is blended with the cold high pressure separator gas from Step b) after first stage Fischer-Tropsch synthesis conversion at a volume ratio (recycle ratio) of 0.5-5, preferably 1-3, and is introduced into the second stage Fischer-Tropsch synthesis reactor. The second stage Fischer-Tropsch synthesis reaction is carried out under the following reaction conditions: the reaction temperature is 200-320° C., preferably 235-275° C.; the reaction pressure is 15-50 bar, preferably 18-38 bar; superficial gas velocity of the reactor inlet is 10-40 cm/s, preferably 15-35 cm/s; the volume ratio of gas flow of the reactor inlet to catalyst is 2000-50000, preferably 5000-30000.

In Step d), top products of the reactor enter a thermal high-pressure separator, and carry out flash separation at 120-220° C., preferably 140-180° C. to obtain gas phase (thermal high-pressure separator gas) and heavy oil phase (thermal high-pressure separator liquid). Gas phase streams enter the cold high-pressure separator, and carry out flash separation again at 5-60° C., preferably 10-50° C. to obtain cold high-pressure separator gas and cold high-pressure separator liquids (blended liquid phase products of light distillate oil and synthetic water). Most of the cold high-pressure separator gas is recycled back to the reactor, and a portion of it is vented out as tail gas. The discharge amount of tail gas is determined by the contents of CO and H₂ in the gas, usually the contents of CO and H₂ are not less than 40%, and preferably not less than 50%. The gas recycled back to the reactor shall be deprived of CO₂ before being blended with cold high-pressure separator gas of the first stage Fischer-Tropsch synthesis. Common CO₂-removing methods such as hot potassium carbonate method or water washing method can be used to remove CO₂ in recycled gases. For the hot potassium carbonate CO₂-removing method, CO₂ absorption column is a common device, the generated potassium bicarbonate desorbs CO₂ in a regeneration column, and the obtained potassium carbonate solution returns to the absorption column again. As for the water washing method, only a water-washing tank needs to be provided.

This process controls the severity of process conditions of the first stage Fischer-Tropsch synthesis, such that the CO conversion rate of the first stage Fischer-Tropsch synthesis ranges at 35%-70%, preferably 40%-65%; the process also ensures space time yield of catalysts, lowers the severity of process conditions of the second stage Fischer-Tropsch synthesis, such that CO one-way conversion rate of the second stage Fischer-Tropsch synthesis is lower than 50%, thus ensuring low methane yield and high oil product yield during the whole process. After combination of the first stage and second stage Fischer-Tropsch synthesis, the total CO conversion rate reaches more than 90%.

Fischer-Tropsch synthesis reactors in Steps a) and c) adopt slurry bed reactors. The first stage Fischer-Tropsch synthesis reactor can be one or a plurality of slurry bed reactors, while merely one slurry bed reactor is provided for the second stage.

The catalysts used in Steps a) and c) can be iron-based or cobalt-based slurry bed catalysts. In case of iron-based catalysts, the volume ratio of H₂ to CO in the fresh synthetic gas is preferably 1.4-1.8. In case of cobalt-based catalysts, the volume ratio of H₂ to CO in the fresh synthetic gas is preferably 2.0. Cobalt-based catalysts are suitable for processing natural gas based synthetic gases, and iron-based catalysts are suitable for processing coal-based synthetic gases.

Main products (hydrocarbon products) produced by the Fischer-Tropsch synthesis device in the present invention are hydrocarbon gases (containing lower olefins), light naphtha and heavy naphtha, diesel distillate, wax and synthetic water, and particularly naphtha, diesel distillate, and wax as main products.

The wax produced in the Fischer-Tropsch synthesis reactor is separated from catalysts through a filter arranged in the reactor, and is then discharged out as products, or it can be obtained by extracting out the slurry in the reactor and separating it from granules of catalysts through a solid-liquid separation device arranged outside. If a filter is arranged outside, the granules of catalysts obtained from separation shall be recycled back to the reactor to continue participating in the reaction.

Compared with the prior art, the two-stage Fischer-Tropsch synthesis process provided in the present invention can bring the following beneficial effects:

A. Compared with the prior art, the two-stage process of the present invention eliminates the recycle gas compressor system of the first stage Fischer-Tropsch synthesis, and thereby reduces equipment investment expenses and relevant operation expenses.

B. In the first stage:

(1) The total conversion rate (i.e. one-way conversion rate) is moderate, the products are reasonably distributed, and the selectivity of methane is low;

(2) All gases of reactor inlet are fresh synthetic gases, being dry and free of moisture, and the total CO conversion rate of the first stage Fischer-Tropsch synthesis is controlled at 35%-75%. Therefore, the partial pressure of gas-phase vapor in the reactor is lower than that when the total CO conversion rate reaches more than 90% in a state of parallel connection, WGS reaction rate is lower than that of parallel connection, and CO₂ productivity and selectivity are both lowered;

(3) Partial pressure of effective gas of the reactor inlet reaches 100%, and under the conditions of slurry bed reactor, maximum fresh synthetic gas is operated at high space velocity, which is rather beneficial to Fischer-Tropsch synthesis reaction and obtaining the most space-time yield.

C. In the second stage:

(1) The cold high-pressure separator gas of the first stage Fischer-Tropsch synthesis is used as a fresh synthetic feed gas of the second stage, which is equivalent to carrying out gas phase condensation dehydration when the reaction of the single-reactor process proceeds halfway in the parallel connection method. This can be understood as the reaction portion of the total CO conversion rate above 65% when partial pressure of gas phase water in the second stage Fischer-Tropsch synthesis reactor is lower than that of a single reactor (can be deemed as the latter half stage of the reactor in case of a single-reactor process in the parallel connection method). Therefore, this is helpful for improving the reaction rate of Fischer-Tropsch synthesis;

(2) As the accumulative conversion rate reaches over 90% comprising the first stage Fischer-Tropsch synthesis reactor, the second stage Fischer-Tropsch synthesis can be carried out with a lower one-way conversion rate than that of parallel connection, and the product distribution can also be improved.

Fresh synthetic gas passes through the first stage Fischer-Tropsch synthesis reactor in one way at a high space velocity, and then the gas phase stream, after being condensed and dehydrated, is introduced into the second stage Fischer-Tropsch synthesis reactor. By this way, the present invention reasonably adjusts the severity of the two stages of technical parameters. This will improve the unit space time yield of catalysts, lower the productivity of methane, and achieve the objects of shorter entire process duration, lower investment, less operation energy consumption, higher catalyst space time yield, and lower methane productivity under the conditions of saving one recycle gas compressor and greatly reducing recycle gas compression operation expenses.

Now the process in the present invention will be further explained in combination with drawings. However, the present invention will not therefore be subject to any limitations. In order to emphasize the idea of the process in the present invention, many necessary devices for industrial application such as heaters, pumps, valves and heat exchangers are omitted in the drawings.

As shown in FIG. 1, after purification and adjustment, a fresh synthetic gas feedstock 1 of CO and H₂ (e.g. volume ratio is 0.67-2.2) exchanges heat with Fischer-Tropsch synthesis products through heat exchangers 105 and 103, and then is introduced into a first stage Fischer-Tropsch synthesis reactor 102 to carry out a Fischer-Tropsch synthesis reaction under the action of Fischer-Tropsch synthesis catalysts. As the Fischer-Tropsch synthesis reaction is a strong exothermic reaction, the reaction heat is taken out using a steam coil, the temperature of the whole reactor is controlled by the pressure of a steam pocket 101, a byproduct steam 26 is produced, the wax generated is separated from the catalyst through a built-in filter in the reactor to obtain a product wax 13, a mixture (i.e. top product) 2 of the feedstock gas after reaction and partial light distillate of Fischer-Tropsch synthesis products comes out from the top of a reactor 102, exchanges heat with the feedstock gas in a heat exchanger 103, and then carries out gas-liquid separation in a thermal high pressure separator 104 to separate a thermal high pressure separator liquid (thermal high pressure separator liquid phase product) 11 and a thermal high pressure separator gas 3; the thermal high pressure separator gas 3 exchanges heat with the feedstock gas, then lowers its temperature to around 50° C., and then carries out gas liquid separation in a cold high pressure separator 106 to separate out a cold high pressure separator gas (unconverted tail gas) 4 and a cold high pressure separator liquid (blended liquid phase products of light distillate oil and water) 6. The cold high pressure separator gas 4 discharged from the top of the cold high pressure separator 106 is blended with a second stage recycled tail gas 35, a blended gas 40 steps up its pressure through a recycle gas compressor, exchanges heat with a top product 33 of the second stage Fischer-Tropsch synthesis reactor through a second stage heat exchanger 115 and a heat exchanger 113, and then is introduced into a second stage Fischer-Tropsch synthesis reactor 112 for Fischer-Tropsch synthesis conversion. Like the first stage Fischer-Tropsch synthesis reactor 102, the reaction temperature of the second stage Fischer-Tropsch synthesis reactor 112 is controlled by a steam pocket 111, and a byproduct steam 25 is produced, the wax generated from the second stage Fischer-Tropsch synthesis is separated from the catalyst through a built-in filter in the reactor to obtain a Fischer-Tropsch synthesis wax 43, which is blended with a first stage Fischer-Tropsch synthesis wax 13 to obtain a wax 88 as the product out of the device. The top product 33 of the second stage Fischer-Tropsch synthesis reactor exchanges heat with the feedstock gas through a heat exchanger 113, and then carries out gas liquid separation in a thermal high pressure separator 114 to separate out a thermal high pressure separator liquid 37 and a thermal high pressure separator gas 34, the thermal high pressure separator gas 34 exchanges heat with the feedstock gas through a heat exchanger 115, then lowers its temperature to around 50° C., and then carries out gas liquid separation in a cold high pressure separator 116 to separate out a cold high pressure separator liquid 36 and a cold high pressure separator gas 10. A portion of the cold high pressure separator gas 10 is discharged as a tail gas 38, the discharge amount is determined by the contents of CO and H₂ in the tail gas to ensue that the contents of CO and H₂ in the tail gas are not less than 40%, when the contents of CO and H₂ in the tail gas are greater than 50%, the discharge amount is decreased; when the contents of CO and H₂ in the tail gas are less than 40%, the discharge amount is increased. Most of the remaining cold high pressure separator gas 27 is introduced into a decarburization system (i.e. a CO₂-removing system) 109, therein the gas contacts a decarburization solvent 5, most CO₂ in the gas enters a decarburization solvent to form a alkali sludge 50 and be discharged out of the decarburization system 109, the CO₂-removed gas as a recycle gas 35 is blended with the cold high pressure separator gas 4 from the first stage Fischer-Tropsch synthesis, and then is introduced into a recycle gas compressor 110 for increasing pressure, the increased blended gas 40 returns to the second stage Fischer-Tropsch synthesis reactor inlet after heat exchange. First stage and second stage cold high pressure separator liquids 6 and 36, after being blended, blend with a thermal low pressure separator gas 39, and then enter a cold low pressure separator 118 to be separated as a Fischer-Tropsch synthetic water 68 comprising oxygen-containing compounds, a Fischer-Tropsch synthesis cold low pressure separator product (light distillate oil) 58, and a cold low pressure separator gas 48 out of the device. The first stage and second stage thermal high pressure separator liquids 37 and 11, after being blended, enter a thermal low pressure separator 117 to be separated as a thermal low pressure separator liquid (heavy distillate oil) 78 and a thermal low pressure separator gas 39, wherein the heavy distillate oil 78 is discharged out of the device as a product, the thermal low pressure separator gas 39, after being cooled (e.g. cooled to 5-50° C.), is blended with cold high pressure separator liquids 6 and 36, and is introduced into the cold low pressure separator 118 for separation.

The Fischer-Tropsch synthesis reaction is a strong exothermic reaction. In order to keep the slurry bed zone of the Fischer-Tropsch synthesis reactor being operated at a constant temperature, a steam coil is arranged in the reactor slurry zone to enable the slurry to exchange heat with the hot water flowing through the steam coil, and a portion of hot water, after absorbing heat, discharges out the reaction heat by the way of isothermal vaporization. The reaction heat of the first stage Fischer-Tropsch synthesis reactor 102 is moved out of the reactor by means of producing the byproduct steam 26 after exchanging with hot water. After exchanging heat with the slurry in the first stage Fischer-Tropsch synthesis reactor, the hot water 23 from the steam pocket 101 forms a mixture of partial hot water isothermal vaporous steam and hot water and returns to the steam pocket 101, wherein the steam 26 is vented after a pressure control by the steam pocket 101, and the liquid level of the steam pocket is maintained by replenishing a hot water 21. Likewise, the reaction heat of the second stage Fischer-Tropsch synthesis reactor 112 is moved out of the reactor by means of producing the byproduct steam 25 after exchanging with hot water. After exchanging heat with the slurry in the second stage Fischer-Tropsch synthesis reactor, the hot water 24 from the steam pocket 111 forms a mixture of partial hot water isothermal vaporous steam and hot water and returns to the steam pocket 111, wherein the steam 25 is vented after a pressure control by the steam pocket 111, and the liquid level of the steam pocket is maintained by replenishing a hot water 22. A replenishing hot water 20 required by the two steam pockets is provided outside the device, and the steam discharged out through pressure control accumulates to become a byproduct steam 28 of the device, and is discharged out of the Fischer-Tropsch synthesis device, or it is used as a heat source.

According to the understanding of the person skilled in the art, the first stage Fischer-Tropsch synthesis reactor 102 shown in FIG. 1 can be one reactor or a plurality of (more than two, e.g. two, three, or four) reactors in parallel; likewise, the second stage Fischer-Tropsch synthesis reactor 112 can be one reactor or a plurality of (more than two, e.g. two, three, or four) reactors in parallel.

The process of the present invention overcomes the defects of high equipment investment and operation expenses, low space time yield, and low CO utilization that exist when synthetic gas is used as a feedstock to produce liquid products using a conventional Fischer-Tropsch synthesis process. In addition, the process of the present invention integrates techniques, simplifies technical process, and is suitable for large-scale industrialized production.

Experiments I. Partial Pressure Experiment of Inlet Effective Gas

The inventor tests the Fischer-Tropsch synthesis reaction performances under partial pressures of inlet effective gases of different reactors using stirred tank slurry bed reaction devices and iron-based catalysts, and the results are shown in Table 1.

TABLE 1 Experiment Results of Partial Pressures of Inlet Effective Gases of Different Reactors Condition No. Condition 1 Condition 2 Technical condition: Gas catalyst ratio of fresh gas, Nml/g/h 8400 7140 H₂/CO ratio of fresh gas, v/v 1.7 1.7 Nitrogen content in fresh gas, mol % 8 8 Tail gas recycle ratio, v/v 2.33 2.75 Superficial gas velocity of reactor inlet, cm/s Benchmark Benchmark Reaction temperature ° C. Benchmark Benchmark Reaction pressure, MPa Benchmark Benchmark Experiment result: Total CO conversion rate, % 75 81 CO₂ selectivity, mol % 13.3 14.7 CH₄ selectivity, mol % 3.6 4.4 CO one-way conversion rate, % 30 32 C₅ ⁺ space time yield, g/g-cat/h 1.0 0.9 Nitrogen content in recycled gas, mol % 18 21

The data in Table 1 shows that, if the superficial gas velocity of reactor inlet, the reaction temperature and the reaction pressure remain the same, when the gas catalyst ratio of fresh synthetic gas decreases from 8400 Nml/g-cat./h in Condition 1 to 7140 Nml/g-cat./h in Condition 2, although the total CO conversion rate can be improved, the space time yield and device productivity are both decreased due to the decrease of effective gas partial pressure of the reactor inlet. The experiment results show that making efforts to improve effective gas partial pressure of reactor inlet is helpful for improving oil productivity of the device, under same superficial gas velocity.

II. Reaction Rate Experiment Under Different Water Partial Pressures

Carry out an experiment based on the following feed gas conditions: CO₂ mole fraction is 0.3, CO mole fraction is 0.2, H₂ mole fraction is 0.4, and total reaction pressure P is 2 MPa. The obtained data on Fischer-Tropsch synthesis reaction rate under different water partial pressures are shown in Table 2:

TABLE 2 Comparison of Fischer-Tropsch Synthesis Reaction Rates under Different Water Partial Pressures Water Water content content in feed in feed gas Reaction gas Reaction phase, rate phase, rate Reaction rate Catalysts Y_(H2O), % r_(FT) Y_(H2O), % r_(FT) improvement, % Fe/Cu/K 0.05 0.043319 0.005 0.048891 11.4% Sediment Fe 0.05 0.006769 0.005 0.012941 91.2% 100Fe/0.3Cu/0.2K 0.05 0.028508 0.005 0.054936 92.7% (sediment) Ruhrchemie LP 0.05 0.014306 0.005 0.027326 91.0% 33/8

As seen from the experiment results in Table 2, although different catalysts have different sensitivities to water partial pressure, decreasing water contents in feed gas can remarkably improve all Fischer-Tropsch synthesis reaction rates. If the feed gas is dehydrated and the water content therein is decreased by 90% from 0.05%, to 0.005% (50 ppm) (volume ratio), the Fischer-Tropsch synthesis reaction rate can increase by 11%-93%. Thus, it can be seen that the effect is very remarkable.

EXAMPLES Example 1

In Example 1, the process shown in FIG. 1 is used. The fresh feedstock gas is a synthetic gas obtained from gasified coal through purification and water gas shift, wherein the volume ratio of H₂ to CO is 1.54, the H₂ content is 60.7%, and the CO content is 39.3% (volume ratio).

Iron-based catalysts are used for both the first stage and the second stage of the present Example, and the catalyst is SFT418-7 developed by Beijing Research Institute of China Shenhua Coal to Liquid and Chemical Co., Ltd and produced by Zhejiang Taide New Material Co., Ltd. Before carrying out in the Fischer-Tropsch synthesis reaction, the catalyst is deoxidized in the reactor.

The Fischer-Tropsch synthesis reaction is carried out under the process conditions listed in Table 3.

Comparison Example 1 Parallel Connection Method

To better demonstrate the benefits of the present invention, the parallel connection method for two sets of single-reactor Fischer-Tropsch synthesis devices that are industrially implemented in a large scale at present is selected for comparison. The process conditions for parallel connection are shown in the last column of Table 3.

TABLE 3 Process conditions of Fischer-Tropsch Synthesis Reaction for Example 1 and Comparison Example 1 Technical method Example 1 (The present Comparison invention) Example 1 First Second (Prior art) Technical condition stage stage Each reactor* Reaction temperature ° C. 255 255 255 Reaction pressure, MPa 2.8 2.8 2.8 Gas catalyst ratio of fresh synthetic 4000 0 2000 gas feedstock, Nml/g-cat./h Gas catalyst ratio of reactor inlet, 4000 4000 4000 Nml/g-cat./h Recycle ratio, v/v 0 0.73 1:1 Superficial gas velocity of reactor 15 15 15 inlet, cm/s Temperature of thermal 160 160 160 high-pressure separator, ° C. Temperature of cold high-pressure 45 45 45 separator, ° C. Temperature of thermal 120 120 120 low-pressure separator, ° C. Temperature of cold low-pressure 40 40 40 separator, ° C. Water content in unconverted tail — 0.005 0.005 gas recycled to second stage Fischer-Tropsch synthesis reactor (%) *Notes: 1) The process conditions of the two reactors in Comparison Example 1 are the same; 2) Gas catalyst ratio is a ratio of volumetric flow of feed gas to mass of catalysts in reactor within a unit time period.

The specific data for substantive effect of the process in the present invention are shown in Table 4.

TABLE 4 Experiment Results Process Example 1 Comparison (The present invention) Example 1 First and (Prior art) First Second second 2 reactors in Results stage stage stages parallel CO one-way conversion 54.7 64.8 59.7 rate, % Total CO conversion 54.7 81.4 91.7 83.6 rate, % CO₂ selectivity (mole 24.7 27.8 25.3 23.5 fraction to converted CO), % CH₄ selectivity (mole 5.1 9.1 6.7 6.6 fraction to total hydrocarbon products), % Hydrocarbon space time 0.445 0.290 0.735 0.705 yield, g/g-cat./h Ratio of totally recycled 0 0.42 0.42 1 gas to fresh synthetic gas, v/v

In Example 1, fresh synthetic gas feedstocks at the time of first stage Fischer-Tropsch synthesis pass through all at once, and CO conversion rate is 54.6%. After the first stage Fischer-Tropsch synthesis reaction, the gas shrinks. To meet the superficial gas velocity of the second stage Fischer-Tropsch synthesis reactor inlet, the recycled gas for the second stage Fischer-Tropsch synthesis shall be replenished, and the recycle ratio is 0.76 v/v. CO one-way conversion rate of the second stage Fischer-Tropsch synthesis is 61.8%, and the total conversion rate reaches 81.4%. The total reaction results after integration of the first and second stages are as follows: total CO conversion rate reaches 91.7%, CO₂ selectivity (mole fraction to converted CO) is 25.3%, CH₄ selectivity (mole fraction to total hydrocarbon generated) is 6.7%, and hydrocarbon space time yield reaches 0.735 g/g-cat./h.

Under the conditions of the same catalyst, the same superficial gas velocity of reactor inlet, and the same reaction temperature and pressure, the integration results of the two sets of single-reactor devices in the parallel connection method are as follows: CO one-way conversion rate is 59.7%, the total conversion rate is merely 83.6%, CO₂ selectivity (mole fraction to converted CO) is 23.5%, CH₄ selectivity (mole fraction to total hydrocarbon generated) is 6.6%, and hydrocarbon space time yield is 0.705 g/g-cat./h. As to the Comparison Example, the recycle ratio of every reactor is 1.0 v/v, viz. two recycle gas compressors are required, and the total amount of recycle gas reaches the amount of fresh synthetic gas. To improve CO utilization rate, it usually requires increasing the total CO conversion rate to over 90%. At this time, it needs decreasing the amount of fresh synthetic gas and increasing the recycle ratio to over 1.5.

According to the results of Examples, CO₂ selectivity and CH₄ selectivity in the process of the present invention are approximately equivalent to those in the conventional prior art when the same amount of fresh synthetic gas is processed; the total CO conversion rate in the present invention is 8% higher than that in the prior art; hydrocarbon productivity of the device (space time yield) is 4% higher than that in the prior art; one recycle gas compressor is decreased; the required amount of recycle gas decreases 58%, and the operation expenses of the gas recycle compression system can be largely reduced.

Example 2

In Example 2, the process shown in FIG. 1 is used. The fresh feedstock gas is a synthetic gas obtained from gasified coal underwent purification and water gas shift, wherein the volume ratio of H₂ to CO is 1.8.

Iron-based catalysts are used for both the first stage and second stage of the present example, and the catalyst is the same as that in Example 1. The process conditions are listed in Table 5.

Example 3

In Example 3, the process shown in FIG. 1 is used. The fresh feedstock gas is a synthetic gas obtained from natural gas, wherein the volume ratio of H₂ to CO is 2.

Cobalt-based catalysts are used for both the first stage and second stage of the present example, and the compositions of the catalysts are 15Co:5Zr:100Al₂C₃. The preparation process is as follows: pouring the alumina baked at 500° C. in atmosphere into water solution of zirconium nitrate in advance in the method of preliminary wetting, carrying out evaporating dehydration and drying with drumming, adding Co(NC₃)₂6H₂O water solution again in the preliminary wetting method, drying, and dipping into cobalt nitrate water solution. Repeating the above process for three times, until the cobalt content in the catalyst predecessor conforms to the requirement, then adding some binder, extruding, pelletizing, drying, baking, and reducing to obtain qualified cobalt-based catalysts. The process conditions are listed in Table 5.

TABLE 5 Process conditions of Examples 2 and 3 Process Example 2 Example 3 (the present (the present invention) invention) First Second First Second Process condition stage stage stage stage Reaction temperature ° C. 240 270 260 210 Reaction pressure, MPa 2.3 2.3 2.3 2.3 Gas catalyst ratio of fresh 7200 0 50000 0 synthetic gas feedstock, Nml/g-cat./h Gas catalyst ratio of reactor 7200 4000 50000 7667 inlet, Nml/g-cat./h Recycle ratio, v/v 0 0.91 0 1.50 Superficial gas velocity of 10 10 40 40 reactor inlet, cm/s Temperature of thermal 140 140 220 220 high-pressure separator, ° C. Temperature of cold 5 5 60 50 high-pressure separator, ° C. Temperature of thermal 60 60 80 80 low-pressure separator, ° C. Temperature of cold 5 5 40 30 low-pressure separator, ° C. Water content in unconverted — 0.003 — 0.01 tail gas recycled to second stage Fischer-Tropsch synthesis reactor (%)

TABLE 6 Experiment Results of Examples 2 and 3 Process Example 2 (the present Example 3 (the present invention) invention) First First and and First Second second First Second second Results stage stage stages stage stage stages CO one-way 64.9 43.8 60.0 37.7 conversion rate, % Total CO 64.9 75.0 91.2 60.0 70.1 88.0 conversion rate, % CO₂ selectivity 33.3 38.6 34.8 0 0 0 (mole fraction to converted CO), % CH₄ selectivity 3.57 4.12 3.7 7.2 8.4 7.58 (mole fraction to total hydrocarbon products), % Hydrocarbon 0.752 0.165 0.917 0.620 0.405 1.025 space time yield, g/g- cat./h Ratio of 0 0.45 0.45 0 0.66 0.66 totally recycled gas to fresh synthetic gas, v/v

If the conventional process wants to reach the total CO conversion rate of the present invention, the amount of the processed fresh synthetic gas shall be reduced, and the recycle ratio shall be increased. Productivity of the device is further decreased, and the compression energy consumption of recycle gas is further increased.

Slurry bed reactors are used as examples in the above examples to describe the present invention. The person skilled in the art knows that the above process and system are also suitable for fixed beds, fixed fluidized beds and fluidized beds after proper adjustment.

Of course, the present invention can also have other specific embodiments. The above is merely preferred embodiments of the present invention but not to limit the present invention. Within the spirit of the present invention, the person skilled in the art may have various alterations and changes based on contents of the present invention. Any alterations and changes thereof should be covered in the protection scope of the present invention. 

1. A two-stage Fischer-Tropsch synthesis process, comprising the following steps: a) a first stage Fischer-Tropsch synthesis reaction introducing a feedstock gas containing CO and H₂ into a first stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts, to obtain products of the first stage Fischer-Tropsch synthesis reaction; wherein the CO conversion rate in the first stage Fischer-Tropsch synthesis reactor is controlled at 30%-70%; b) separation of the products of the first stage Fischer-Tropsch synthesis reaction separating the products of the first stage Fischer-Tropsch synthesis reaction, so as to separate water from the unconverted tail gas and to obtain hydrocarbon products and an unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction; c) a second stage Fischer-Tropsch synthesis reaction introducing the unconverted tail gas obtained from Step b) into a second stage Fischer-Tropsch synthesis reactor to carry out a Fischer-Tropsch synthesis reaction under the action of catalysts, to obtain products of the second stage Fischer-Tropsch synthesis reaction; d) separation of the products of the second stage Fischer-Tropsch synthesis reaction separating the products of the second stage Fischer-Tropsch synthesis reaction, so as to separate water from the unconverted tail gas and to obtain hydrocarbon products and an unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction, with a portion of the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction being returned to the second stage Fischer-Tropsch synthesis reactor for recycle reactions, wherein the unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction is not returned to the first stage Fischer-Tropsch synthesis reactor for recycle reactions; in Step a), the fresh synthetic gas as the feedstock gas passes through the first stage Fischer-Tropsch synthesis reactor in one way, and wherein the separations in Steps b) and d) comprise oil-water-gas separations of top products of the Fischer-Tropsch synthesis reactions.
 2. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein the volume ratio of CO to H₂ in the feedstock gas is 0.67-2.2.
 3. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein the oil-water-gas separation of the top product of the first stage Fischer-Tropsch synthesis reaction in Step b) and/or the oil-water-gas separation of the top product of the second stage Fischer-Tropsch synthesis reaction in Step d) comprise the following steps: firstly, carrying out a flash separation with thermal high pressure separators to obtain thermal high pressure separator liquids and thermal high pressure separator gases; then, carrying out a flash separation for the thermal high pressure separator gases using cold high pressure separators to obtain two phases: the cold high pressure separator liquids being blended liquid phase products of light distillate oil and water, and the cold high pressure separator gases being the unconverted tail gases.
 4. The two-stage Fischer-Tropsch synthesis process according to claim 3, further comprising: e) introducing the thermal high pressure separator liquids of the first stage and second stage Fischer-Tropsch synthesis reactions to a thermal low pressure separator, carrying out a flash separation to obtain heavy distillate oil products as a thermal low pressure separator liquid and a thermal low pressure separator gas; and f) introducing the cold high pressure separator liquids of the first stage and second stage Fischer-Tropsch synthesis reactions and the optional thermal low pressure separator gas to a cold low pressure separator, carrying out a flash separation to obtain a cold low pressure separator gas, a light distillate oil product, and a water.
 5. The two-stage Fischer-Tropsch synthesis process according to claim 3, wherein: The thermal high pressure separators is operated at 120-220° C.; The cold high-pressure separators is operated at 5-60° C.
 6. The two-stage Fischer-Tropsch synthesis process according to claim 4, wherein: The thermal low-pressure separator is operated at 60-200° C.; The cold low-pressure separator is operated at 5-60° C.
 7. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein the CO conversion rate in the first stage Fischer-Tropsch synthesis reactor is controlled at 40%-65%.
 8. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein the first stage Fischer-Tropsch synthesis reaction in Step a) and/or the second stage Fischer-Tropsch synthesis reaction in Step c) are carried out under the following reaction conditions: The reaction temperature is 200-320° C.; The reaction pressure is 15-50 bar; The superficial gas velocity of reactor inlet gas is 10-40 cm/s; The ratio (i.e. gas catalyst ratio) of volumetric flow of reactor inlet gas to the mass of catalyst is 2000-50000 Nml/g-cat./h.
 9. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein the first stage Fischer-Tropsch synthesis reactor and the second stage Fischer-Tropsch synthesis reactor is one slurry bed reactor or a plurality of slurry bed reactors in parallel.
 10. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein the volume ratio of CO to H₂ in the feedstock gas is 1.4-2.
 11. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein, the catalysts used in Steps a) and c) are iron-based or cobalt-based catalysts.
 12. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein, the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction is blended with all unconverted tail gases of the first stage Fischer-Tropsch synthesis reaction, then is introduced into the second stage Fischer-Tropsch synthesis reactor.
 13. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein, the feedstock gas in Step a) is a synthetic gas underwent purification and water gas shift.
 14. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein, the water content in the unconverted tail gas entering the second stage Fischer-Tropsch synthesis reactor is less than 0.05% by volume.
 15. A two-stage Fischer-Tropsch synthesis system, comprising: A) a first stage Fischer-Tropsch synthesis reactor, containing Fischer-Tropsch synthesis catalysts, the first stage Fischer-Tropsch synthesis reactor at least comprising: a first stage reactor inlet, located at the bottom of the first stage Fischer-Tropsch synthesis reactor; a first stage reactor top outlet, located at the top of the first stage Fischer-Tropsch synthesis reactor; a first stage Fischer-Tropsch synthesis wax or slurry outlet, located at a slurry bed zone of the first stage Fischer-Tropsch synthesis reactor; B) a first stage separation system, for separating a top product from the first stage reactor top outlet, thereby separating water from the unconverted tail gas to obtain hydrocarbon products and unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction; the first stage separation system comprising: a first stage separation system inlet, connected with the first stage reactor top outlet; a plurality of first stage separation system outlets, comprising: a first stage hydrocarbon product outlet, and a first stage unconverted tail gas outlet; C) a second stage Fischer-Tropsch synthesis reactor, containing Fischer-Tropsch synthesis catalysts, the second stage Fischer-Tropsch synthesis reactor at least comprising: a second stage reactor inlet, located at the bottom of the second stage Fischer-Tropsch synthesis reactor, and connected with the first stage unconverted tail gas outlet; a second stage reactor top outlet, located at the top of the second stage Fischer-Tropsch synthesis reactor; a second stage Fischer-Tropsch synthesis wax or slurry outlet, located at a slurry bed zone of the second stage Fischer-Tropsch synthesis reactor; D) a second stage separation system, for separating a top product from the second stage reactor top outlet, thereby separating water from the unconverted tail gas to obtain hydrocarbon products and unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction, the second stage separation system comprising: a second stage separation system inlet, connected with the second stage reactor top outlet; a plurality of second stage separation system outlets, comprising: a second stage hydrocarbon product outlet, and a second stage unconverted tail gas outlet; wherein the first stage unconverted tail gas outlet is not connect with the first stage reactor inlet, such that the fresh synthetic gas as the feedstock gas passes through the first stage Fischer-Tropsch synthesis reactor in one way; wherein B) the first stage separation system and/or D) the second stage separation system comprise an oil-water-gas separation device.
 16. The two-stage Fischer-Tropsch synthesis system according claim 15, wherein the first stage Fischer-Tropsch synthesis reactor and the second stage Fischer-Tropsch synthesis reactor is one slurry bed reactor or a plurality of slurry bed reactors in parallel.
 17. The two-stage Fischer-Tropsch synthesis system according claim 15, wherein the number of the first stage Fischer-Tropsch synthesis reactors is greater than or equal to the number of second stage Fischer-Tropsch synthesis reactors.
 18. The two-stage Fischer-Tropsch synthesis system according claim 17, wherein oil-water-gas separation systems of B) the first stage separation system and/or D) the second stage separation system comprise: thermal high pressure separator(s), comprising: a thermal high pressure separator inlet, connected with the first stage separation system inlet or the second stage separation system inlet, a thermal high pressure separator liquid outlet, and a thermal high pressure separator gas outlet; cold high pressure separator(s), comprising: a cold high pressure separator inlet, connected with the thermal high pressure separator gas outlet, a cold high pressure separator liquid outlet, and a cold high pressure separator gas outlet.
 19. The two-stage Fischer-Tropsch synthesis system according claim 18, further comprising: a thermal low pressure separator, comprising: a thermal low pressure separator inlet, connected with the thermal high pressure separator liquid outlet(s) of the first stage separation system and/or the second stage separation system, a thermal low pressure separator gas outlet, a thermal low pressure separator liquid outlet; optionally, a cold low pressure separator, comprising: a cold low pressure separator inlet, connected with the thermal low pressure separator liquid outlet and/or the cold high pressure separator liquid outlet, a cold low pressure separator gas outlet, a light distillate oil outlet, a Fischer-Tropsch synthesis water outlet.
 20. The two-stage Fischer-Tropsch synthesis system according claim 18, further comprising a CO₂-removing system, the CO₂-removing system comprising: a CO₂-removing solvent inlet, a CO₂-removing solvent outlet, a CO₂-removing system gas inlet, connected with the cold high pressure separator gas outlet, a CO₂-removing system gas outlet, connected with the second stage reactor inlet.
 21. The two-stage Fischer-Tropsch synthesis system according to claim 15, wherein the first stage Fischer-Tropsch synthesis reactor is one slurry bed reactor or a plurality of slurry bed reactors in parallel, and the second stage Fischer-Tropsch synthesis reactor is one slurry bed reactor.
 22. The two-stage Fischer-Tropsch synthesis system according to claim 15, further comprising: a wax filter, arranged inside or outside the first stage Fischer-Tropsch synthesis reactor and the second stage Fischer-Tropsch synthesis reactor.
 23. The two-stage Fischer-Tropsch synthesis process according to claim 1, wherein, when iron-based catalysts are adopted, the volume ratio of H₂ to CO in the feedstock gas is 1.4-1.8; when cobalt-based catalysts are adopted, the volume ratio of H₂ to CO in the feedstock gas is 1.8-2.2.
 24. The two-stage Fischer-Tropsch synthesis process according to claim 12, wherein the blend volume ratio of the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction to the unconverted tail gas of the first stage Fischer-Tropsch synthesis reaction is 0.5-5.
 25. The two-stage Fischer-Tropsch synthesis process according to claim 12, wherein CO₂ in the unconverted tail gas of the second stage Fischer-Tropsch synthesis reaction is removed before the blending. 